In many industrial processes, mixtures of air and condensible vaporous materials such as gasoline, halogenated hydrocarbon solvents and the like, are produced. The recovery of the condensible vaporous materials for disposal or reuse is ordinarily an economic and ecological necessity.
In the past, recovery has been effected by the use of a solid adsorbent or desiccant such as activated carbon. The usual adsorption system of this type comprises one or more adsorber vessels containing the adsorbent arranged so that gasoline or solvent vapor-laden air to be treated travels through a bed of the adsorbent whereby the gasoline or solvent vapors are adsorbed and the thus-purified air is exhausted to the atmosphere. When the adsorbent has become substantially saturated with the gasoline or solvent, the influx of the vapor laden air is terminated, and the adsorbed material is driven off by heating and/or applying a vacuum to the absorbent bed. The solvent is then recovered by passing the effluent stream into suitable condensing apparatus.
Where live steam is the sole source of heat as in U.S. Pat. No. 2,211,162 and no vacuum is applied, an additional drying step for the adsorbent material is necessary thereby increasing the duration of a complete cycle. Where live steam is used in combination with a vacuum to achieve desorbtion, the steam condenses as water on the adsorbent bed, the temperature of the bed rapidly falls below the vaporization temperature of the condensed vapor and complete recovery can not be effected. A similar result (i.e., rapid decrease of bed temperature during recovery resulting in incomplete removal of the condensed vapors) occurs when the adsorbent bed is heated other than with live steam and a vacuum is applied. The adsorbent material has a low specific heat and does not act as an effective heat sink to counteract the loss of heat while a vacuum is being drawn.
Incomplete recovery of the condensed material leads to the possible release of the material to the atmosphere during the next adsorption cycle, more frequent regenerations and periodic regeneration of the adsorbent at relatively high temperatures without recovery of the condensed material. Thus, recovery methods as practiced by the prior art are inefficient and may be accompanied by pollution of the environment. Vapor recovery systems heretofore proposed are illustrated by U.S. Pat. Nos. 1,717,103; 2,211,162; 2,975,860; 3,225,516; 3,355,860; and 3,883,325.